As semiconductor devices are now manufactured using 100 nm technology or less, resistance in wiring layers, such as word lines that constitute a gate electrode, has significantly increased. This increase in resistance may result in, for example, signal delays in the wiring layers. Accordingly, to address the problem of increased resistance in the wiring layers, a polymetal gate electrode including a conductive polysilicon layer, a barrier film, and a metal film sequentially formed on a semiconductor substrate has been suggested.
Such a polymetal gate electrode structures is discussed in, for example, U.S. Pat. No. 6,774,442. As discussed therein, the barrier film of the polymetal gate mainly includes tungsten nitride (WN) or tungsten silicon nitride (WSiN). When the barrier film includes Wn or WSiN, the word lines may have a low resistance, but may also have low thermal stability. Therefore, the barrier film may be partially damaged in a selective oxidation process performed after the word lines have been patterned. The selective oxidation process may restore the word lines damaged during patterning by annealing the word lines at a temperature of at least about 800° C. Thus, metal silicide may be generated from a reaction between the silicon (Si) of the polysilicon layer and a metal of the metal film.
Referring now to FIGS. 1A and 1B, scanning electron microscope (SEM) images illustrating results of annealing a conventional polymetal structure having a barrier film formed of WN will be discussed. In particular, FIG. 1A is a SEM image of a cross-section of a conventional polymetal structure. As illustrated therein, the structure includes a gate oxide film (Gox) on a Si substrate, a polysilicon layer (poly-Si) on the gate oxide film, a WN barrier film on the polysilicon layer and a W film on the WN layer. The WN film may have a thickness of about 50 Å and the W film may have a thickness of about 400 Å.
Referring now to FIG. 1B, a SEM image of a cross section of the conventional polymetal structure of FIG. 1A after having been annealed will be discussed. In particular, the polymetal structure of FIG. 1A was annealed at a temperature of about 850° C. for about 40 minutes to obtain the structure illustrated in FIG. 1B. As illustrated in FIG. 1B, the WN film between the polysilicon layer and the W film is decomposed into W and N, and has been partially damaged by the annealing process. Furthermore, the majority of the polysilicon layer and the W film have been silicided.